In semiconductor manufacturing, plasma ashing is the process of removing the photoresist from an etched wafer. Plasma in this context is an ionized form of a gas. A gas ionizing apparatus, also referred to as a plasma generator, produces a monatomic reactive species of oxygen or another gas required for the ashing process. Oxygen in its monatomic or single atom form, as O rather than O2, is the most common reactive species. The reactive species combines with the photoresist to form ash which is removed from the work piece with a vacuum pump.
Typically, monatomic oxygen plasma is created by exposing oxygen gas (O2) to a source of energy, such as a RF discharge. At the same time, many charged species, i.e. ions and electrons, are formed which could potentially damage the wafer. Newer, smaller circuitry is increasingly susceptible to damage by charged particles. Originally, plasma was generated in the process chamber, but as the need to avoid charged particles has increased, some machines now use a downstream plasma configuration, where plasma is formed remotely and channeled to the wafer. This reduces damage to the wafer surface.
Monatomic oxygen is electrically neutral and although it does recombine during the channeling, it does so at a slower rate than the positively or negatively charged particles, which attract one another. Effectively, this means that when substantially all of the charged particles have been neutralized, the reactive neutral species remains and is available for the ashing process.
Current plasma generating apparatus present a variety of challenges during ashing procedures. Generally, plasma is generated using a coil, often copper, wrapped around a dielectric tube, such as quartz or aluminum/sapphire tube. The coil is energized with a radio frequency (RF) voltage from an appropriate RF generator. Plasma formation is initiated by capacitively coupling the electric field through the quartz to the rarefied gas inside the quartz tube. As the power level and current through the coil are increased, the plasma switches from a capacitively coupled mode to an inductively coupled mode. Significant voltages exist on the coil. Difficulties arise in trying to isolate the high voltage components to prevent these components from breaking down and arcing to cause damage to other components. In addition, the high voltages generate a high electric field across the quartz and can cause significant ion bombardment and sputtering on the inside of the quartz tube thus reducing its lifespan and increasing its maintenance needs. A reduction in the ion bombardment energy may be helpful.
In addition, as illustrated in schematic cross section in FIG. 1, prior art plasma sources 10 for ashing have smaller diameter plasma generation regions 12, in quartz cylindrical containers 15, than the work pieces 20 that are to be treated. Accordingly, plasma flows from a smaller diameter plasma generation region 12 of the quartz cylinder 15 of about 76 mm diameter that is surrounded by a RF induction coil 14, to a larger diameter distribution region 16 of a diameter approximating the work piece diameter, often about 300 mm diameter. In the distribution region 16, the oxygen atoms (O), which are the desired product in the plasma generator effluent, are spread out or dispersed over a larger cross sectional area than that of the generation region 12 in an attempt to control the flux of O atoms to the surface of a work piece 20. In addition, the distribution region 16 includes a diffuser 18 of some kind to further facilitate a desired plasma distribution over the surface of the work piece 20. Significant numbers of O atoms are lost in this process.
Ion bombardment of the quartz cylinder 15 poses another significant challenge. When a small diameter plasma source 10 is used, the plasma density should be very high in order to generate enough O atoms to perform ashing at an acceptable rate. This high plasma density coupled with the high energy fields (E-fields) present in the coil 14 cause significant ion bombardment of the quartz container 15 and a reduced container lifespan. One method to ameliorate this effect is to place a Faraday shield 22 between the quartz container 15 and the coil 14, as illustrated in the schematic cross section of FIG. 2. This effectively prevents the E-fields from penetrating the quartz container 15 and consequently reduces the sputtering of the quartz container 15. The addition of the Faraday shield 22 reduces one problem at the expense of creating additional problems. The Faraday shield 22 is complex, increases cost, requires water cooling and consumes power that would otherwise be delivered to the plasma.
In addition, present day plasma generator apparatus suffer from non-uniform plasma production. Generally, when an oxygen-containing gas flows through the container, plasma generation is initiated in the tube adjacent the coil. But since the E-field has limited penetration into the container, the peak area for energy dissipation is near the inner wall of the container. Due to this limited penetration of the E-field, the plasma forms a ring 25 inside the quartz container 15, as seen from above, and as schematically shown in FIG. 3, with the area of peak power dissipation being near the inner wall of quartz container 15. There is a hole 26 corresponding to a nearly field-free region where there is little or no energy dissipation from the excitation fields. For example, in the 76.2 mm diameter tube on the Gamma 2130™ of Novellus Systems, Inc. [San Jose Calif.], the size of the central hole 26 in the ring 25 is small, although quite visible under certain conditions. While gas flows through the entire cross section of the quartz container 15, oxygen in the gas flow is mainly dissociated in the ring 25 to produce 0 atoms. Very little of the oxygen in the remainder of the gas flow is dissociated to 0 atoms. Accordingly, a large portion of the incoming gas flow, namely gas in the vicinity of the center of the cylindrical gas flow in container 15, is not subjected to sufficient energy for ionization.
In addition, present day plasma generators are difficult to adapt to ashing larger wafers. If the quartz container 15 is increased in diameter, the peak plasma region remains approximately the same size and is still located near the wall. The hole 26 in the ring 25 increases in size dramatically as the diameter of the quartz container 15 is increased. The majority of the gas flows down the center of the quartz container 15 and is never directly ionized. Thus, few O atoms are produced in the central region of the quartz container 15. The efficiency of producing O atoms in larger diameter quartz containers is therefore expected to be low.
Accordingly, it is desirable to provide an improved plasma generation apparatus that is suitable for use in ashing procedures in semiconductor fabrication. It is also desirable to provide an apparatus that is able to provide a more uniform distribution of O atoms over a large diameter work piece, such as a 300 mm or larger wafer. It is further desirable to provide a plasma generator that does not require Faraday shields, but that also provides an acceptable quartz container lifespan. In addition, it is desirable to provide a plasma generation apparatus and/or process that converts oxygen more efficiently to O atoms. Other desirable features and characteristics of the present technology will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.